It has been common for many years to construct the cylinder housing for the majority of reciprocating engines of at least two pieces, a block and a head, each piece being cast of ferrous material in a sufficiently heavy and rugged configuration to provide a wide margin of safety against thermal cracking without serious regard to engine weight and energy dissipation. There has now been a recent movement to employ aluminum as a casting material for either said head or block or both. This movement is a natural outgrowth of the desire to improve fuel economy for a vehicle by measures which reduce weight. The savings in weight by use of aluminum is obvious and inviting. Employment of aluminum has lead to some changes in the method of constructing the head, but the design and mechanical configuration of the head have changed little as a result of the material substitution. Aluminum components can be cast by one of several different modes, each having their advantages and disadvantages. The earliest conventional mode was to use a typical sand casting technique; sand casting restricts the aluminum alloy selection to that which will develop proper dispersed precipitation particles at a slower chill rate or solidification rate, characteristic of sand casting. Some casters have turned to high pressure die-casting or permanent molding techniques which permit the employment of more advanced aluminum alloys; however, sand cores cannot be utilized with these methods and thus the freedom to design internal passages is restricted. In addition, each of these methods require from 1.5 to as much as three times the molten metal for the finished casting. High pressure die-casting usually requiring impregnation of the resultant casting, an expensive procedure.
Whether dictated by casting method or mechanical design, neither the wall thickness or wall arrangement of the castings have been appreciably reduced by virtue of the aluminum substitution and thus remain a common disadvantage. Nor have the engines employing components with substituted aluminum exhibited a worthwhile improvement in horsepower, engine efficiency and a reduction in emissions.